Unless otherwise indicated herein, the description in this section is not prior art to the claims in this application and is not admitted to be prior art by inclusion in this section.
To achieve excellent development property when forming an image by electrophotographic method, it is necessary to keep a toner amount in a developer housed in a developer housing portion within an appropriate range. In view of this, general developer housing portions internally include a stirring unit and a sensor. The stirring unit stirs the developer to prevent the developer from accumulating and attaching. The sensor measures the toner amount of the developer. When the toner amount measured by the sensor reaches to equal to or less than a predetermined replenishment threshold, replenishing the toner in the developer housing portion ensures keeping the toner amount in the developer within the appropriate range.